Scanners and copiers use document feeders to transport documents into the machine. Mechanisms used for the transportation of documents, including paper or sheets of other material, have the capacity to accidentally pick up more than one document fed from a stack of documents. It is necessary to determine when more than one document is pulled into a document transport since multiple documents may jam the transport or prevent processing some documents.
A non-contact method for multiple document detection sends ultrasound signals through the document stream to determine if more than one document is present. Sending ultrasound through paper results in attenuation of the ultrasound signal. It is possible to determine the presence of multiple documents by measuring the phase shift ultrasound signal passing through documents. See U.S. Pat. No. 4,066,969 which is herein incorporated by reference. As ultrasound passes from the ultrasonic transmitter to the ultrasonic receiver, the phase changes, depending on the wavelength and the distance.
Due to vibrations, temperature changes, or usage, the distance between a transmitter and a receiver can change over time. It is often desirable to have the signal at the receiver in phase with the signal at the transmitter. In order to make this happen, the distance between the transmitter and receiver must be physically changed by moving one or the other or both. This requires moving parts that require physical space and that will eventually wear out. It would be highly desirable to calibrate ultrasonic detectors electronically.